1. Field of the Invention
The present invention, in the use of the ozonated water for such purposes as cleaning and giving a surface treatment, concerns a method for supplying the ozonated water with the ozone concentration maintained constant even when the amount of the ozonated water to be used at the place of use is varied. To be specific, the present invention is used in producing and supplying the ozonated water by following the fluctuations in the amount of the ozonated water to be used at the place of use. Examples of this use are when the ozonated water is utilized for such operations in the process for the production of semiconductors as treating substrates by peeling a resist from a substrate or cleaning the substrate after removal of the resist and such operations as giving a surface treatment, cleaning, and sterilizing ordinary resins and metals.
More specifically, the present invention concerns a method for the supply of an ozonated water which can be applied in the treatment of semiconductor substrates and liquid crystal substrates to the removal of resist scraps and organic stains, the removal of defiling metals and extraneous substances, and the removal of organic and inorganic dirt and extraneous substances from the masking materials made of quartz plate and used in the production of semiconductors and liquid crystals. In the ozone treatment of resins and metals, the method of this invention is available for treating by surface oxidation, cleaning, and sterilizing resins and for removing organic substances from the surfaces of metals and treating the metals by oxidation, for example.
2. The Prior Art
By reason of the low load exerted by the strong oxidizing power of the ozonated water and the self-decomposing property of the ozone on the environment, the utilization of the ozonated water has been promoted in various fields. The treatment using the ozonated water is effected by various methods such as directly introducing the ozonated water supplied from the ozonated water producing unit into the treating column holding an article under treatment or causing the ozonated water to flow directly onto an article under treatment. Particularly, in the treatment which utilizes the strong oxidizing power of the ozonated water, the temperature, treating time, concentration of the ozonated water, flow rate of the ozonated water, and method of the treatment form important factors for the sake of acquiring a fixed quality of treatment. While the temperature and the treating time are capable of accomplishing prescribed conditions by the control in the treating unit, the concentration of the ozonated water and the flow rate of the ozonated water depend on the capacity of the ozonated water producing unit.
The ozone in water manifests a high self-decomposing property. Particularly, when it has a high concentration exceeding 10 ppm, this concentration is conspicuously decreased by the unit. When semiconductors and resins are treated by using the ozonated water, the concentration of the ozonated water must be maintained constant for the sake of stably obtaining products surpassing a fixed level.
Thus, even when the ozonated water is used intermittently at the place of use, it has been customary for the purpose of maintaining the concentration of the ozonated water constant not to suspend the ozonated water. This occurs even during the time interval having no use for the ozonated water but to allow the ozonated water to flow past the place of use into the waste bin. Recently, as the utilization of the ozonated water has gained in impetus, in the place of the conventional practice of having ozonated water producing units disposed severally at different places of use, the feasibility of a system having a large ozonated water producing unit disposed at the center and forwarding the produced ozonated water to different places of use separated by distances from the producing unit has been tried.
In the circumstance, an attempt simply to forward the ozonated water or circulate it randomly results in excess use of the ozone and the feed water and excess design of ozonated water production facilities. These adverse effects are aggravated in accordance as the concentration of the ozonated water to be used increases.
Patent Document 1 discloses a method which aims to maintain the concentration across a long distance without using an ozone concentration meter by forming a circuit system and estimating the amount of decomposition of ozone during the conveyance over the distance based on the rate of ozone decomposition. Patent Document 2 discloses an ozonated water supply device which takes account of the attenuation of the concentration over the distance similarly in the circulation system and has a mention of occasionally using the ozonated water of high concentration as diluted at the place of use.
[Patent Document 1] Official Gazette of JP-A-0 7-277705 (page 2 and FIG. 1)
[Patent Document 2] Official Gazette of Patent No. JP3321557 (pages 1-3)
Since these prior techniques cover the concentrations of ozonated water in a range not exceeding about 20 ppm at most and since the degrees of purity of the water used thereby do not seem to assume the purity of the level of the ultra pure water used currently in semiconductors, it is inferred that the attenuation of the ozone is not so large. Also the emulation of a certain degree is attainable without requiring particularly rigid control even when the ozonated water is used intermittently. That is, the circulation of the ozonated water for the sake of recovery of the ozonated water is an effective measure where the attenuation of the concentration of the ozonated water due to the elapse of time is small. When the rate of attenuation is large as in the case of the ozone in the ultra pure water, the attenuation of the ozone in the return flow of the circulation is largely affected by the fluctuation of the amount of the ozonated water used at the place of use. By the mere circulation, therefore, it has been difficult to comprehend the amount of the ozonated water used and the concentration of ozone in the return flow and it has been difficult to attain constant supply of the ozonated water of the constant concentration.
When the amount of the ozonated water to be used at the place of use is constant, the ozone concentration can be controlled so that it may remain constant at a remote place by using the method taught by Patent Document 1 or 2. Further, even in the intermittent use, it may be possible to repress the decrease of the ozone concentration in the return flow to a small amount. This is done by setting the flow rate of the circulation of the ozonated water at a sufficiently large magnitude as compared with the maximum magnitude of the amount of the ozonated water to be used.
In the manufacture of a plant unit, it is commendable on account of the necessity for cutting the cost to set the flow rate of the circulation of the ozonated water at a proper magnitude conforming to the maximum amount of use. Where the use at the place of use is intermittent, the ozone concentration in the return flow tends to fluctuate largely proportionately to the amount of use.
In forming a circulating line in an ordinary plant unit, the practice of installing a storage tank for the return flow, receiving the water supplied therein, and supplying the water to the target plant unit is often followed with a view to promoting the stabilization of the operation. For the control of the water supplied here, the method for fixing the liquid level in the storage tank is popularly adopted. When this method is intended to be adopted in the present system, the ozone gas to be supplied to the ozonated water production device necessitates simultaneous production and supply of the same amount of the ozone gas as the amount of the ozonated water used at the place of use, namely the amount of the ozone to be consumed. The cost of the ozone gas generating unit occupies a large proportion in the cost of the manufacture of the ozonated water producing unit. For the purpose of constructing the system economically as a whole, it is commendable to lower the cost of the ozone gas generating unit to the fullest possible extent and complete the unit in a small scale.
The supply of the constant-concentration ozonated water entails numerous factors as mentioned above. These factors may be ascribed to the fact that the ozonated water decomposes at a very high speed and the fact that the expense of the ozone gas generating unit assumes a large proportion of the expense of the entire system. Thus, the desirability of developing a method for effective supply of the ozonated water satisfying all these points of view has found a widespread recognition.